All Astronautical Evolution posts in 2013:

Neubrandenburg Thoughts (I): OldSpace versus NewSpace (Nov.)

Highlights from the Starship Century Symposium in London (Nov.)

Alien Civilisations: Two Competing Models (Oct.)

Elysium, Earth; Elysium, Mars (Sept.)

The Futures We Love to Fear (Aug.)

Quotations from Sophie’s World (May)

Do I Really Exist? (May)

When will Voyager 1 leave the Solar System? (April)

Technological Singularity, or Plateau? The case for antisingularitarianism (March)

Space and Sustainability: Ecological Collapse versus Technological Growth (Feb.)

Manned spaceflight on the plateau awaits a new business model (Jan.)

New in 2020:

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AE posts:

2022: What’s to do on Mars?…

2021: New space company Planetopolis…

2020: Cruising in Space…

2019: The Doomsday Fallacy, SpaceX successes…

2018: I, Starship, atheism versus religion, the Copernican principle…

2017: Mars, Supercivilisations, METI…

2016: Stragegic goal for manned spaceflight…

2015: The Pluto Controversy, Mars, SETI…

2014: Skylon, the Great Space Debate, exponential growth, the Fermi “paradox”…

2013: Manned spaceflight, sustainability, the Singularity, Voyager 1, philosophy, ET…

2012: Bulgakov vs. Clarke, starships, the Doomsday Argument…

2011: Manned spaceflight, evolution, worldships, battle for the future…

2010: Views on progress, the Great Sociology Dust-Up…

Chronological index

Subject index

General essays:

Index to essaysincluding:

Talk presented to students at the International Space University, May 2016

Basic concepts of Astronautical Evolution

Options for Growth and Sustainability

Mars on the Interstellar Roadmap (2015)

The Great Sociology Debate (2011)

Building Selenopolis (2008)


Issue 95, 1 October 2013 – 44th Apollo Anniversary Year

=============== AE ===============

Alien Civilisations: Two Competing Models

Stephen Ashworth, Oxford, UK

Speculations about the existence of extraterrestrial civilisations analogous to our own fall naturally into two broad camps, which we may for convenience describe as the Steady State model versus the Big Bang model (not to be confused with the cosmological theories of the same names). There is also a Hybrid model which combines the two in true Hegelian fashion (thesisantithesissynthesis).

First noted in comments of mine on the Centauri Dreams blog (Astrobiology: Enter the Seager Equation, 11 Sept. 2013), an article expanding this point of view first appeared as a post on Centauri Dreams (Alien Civilisations: Two Competing Models, 18 Sept. 2013), before being revised and republished here.

The Steady State model

This is the basis of the famous Drake equation. Drake assumed that for a long time in the past, and for a long time to come, civilisations have been and will continue to be coming into existence, persisting for a while and then vanishing again. The question which interested him was whether the rate of appearance of civilisations capable of interstellar radio communication, and their average longevity, were large enough to make it statistically likely that we would be able to establish contact with a nearby alien society before either they or we became extinct.

Drake regarded civilisations as entirely sedentary or static phenomena. Thus the locations at which they might be found today are always the same as those at which they originally evolved from their biological forebears, and thus closely resemble our planet Earth. They must be found orbiting closely Sun-like stars in circular orbits, thus in the so-called “habitable zone” (planets outside the zone where surface liquid water is possible presumably being habitable only by creatures who were not capable of radio astronomy, or of changing the atmospheric chemistry if there was no atmosphere, and therefore undetectable by astronomical means).

Steady state diagram

Diagram 1 shows schematically how many civilisations exist at any one time in the Galaxy on the Steady State model. For simplicity it is assumed that each star has either one civilisation, or none. The total number of stars continues to increase slowly as long-lived dwarf stars are added to the population. The number of civilisations rises slightly faster, as longer-lived planets come into play. We are now at point A on the time axis.

The number of stars occupied at any one time is a small fraction of the total (the diagram exaggerates the fraction for clarity). For example, if we share the Galaxy with a million other civilisations at the present time, as optimists may hope, then only 0.00001 of the stars are currently occupied.

All such civilisations arise independently of one another. Collapsed civilisations are not replaced on their planet of origin, but are replaced by other civilisations arising elsewhere. Civilisations are randomly scattered throughout the Galaxy, though Gonzalez, Brownlee and Ward have presented arguments as to why the centre and the outlying galactic regions may be less hospitable than a ring partway out from the centre, where indeed our own Solar System is found.

Civilisations remain completely dependent upon their planet of origin, and the distance between nearest-neighbour planets of that type (perhaps tens of light-years, but quite possibly more – in their 1984 worldship paper Bond and Martin estimated 140 light-years) forbids interstellar settlement.

The Big Bang model

In his book Contact with Alien Civilisations, Michael Michaud reviews the ideas of a number of people who have gone beyond the Drake equation by taking account of the possibility of interstellar colonisation, including Freeman Dyson and Seth Shostak. A similar view has been taken by Ian Crawford, whose article in Scientific American a few years ago discussed the prospect of a dynamic civilisation colonising the entire Galaxy by star-hopping.

Using technologies readily conceivable today (such as nuclear fusion rocket propulsion), a wave of colonies from an expanding civilisation in our Galaxy might take 1,000 years to make each 5 light-year jump (say, travelling for 500 years at 1% of light speed, then taking another 500 years to build up sufficient infrastructure at the target system to make the next jump). Since the Galaxy is about 100,000 light-years across, such a civilisation could spread daughter civilisations to every suitable star and planetary system in the Galaxy within 20 million years.

This, however, is only 0.2% of the age of the Galaxy. The introduction of faster ship speeds makes absolutely no difference: even without warp drive or FTL travel, on a cosmological timescale such a transition from civilisation nowhere to civilisation everywhere is, as Crawford demonstrated, essentially instantaneous.

For a long time, then, the Galaxy is completely devoid of intelligent life. But then one civilisation appears, and spreads throughout the Galaxy in a burst of expansion which we are calling the Big Bang. Thereafter, the locations at which intelligent, technological life is found are almost all colonies, and that life is ubiquitous and permanent.

Big bang diagram

Diagram 2 shows schematically how many civilisations exist at any one time in the Galaxy on the Big Bang model. If humanity is alone, then we are at point B. But there is a possibility (though a small one) that another civilisation in our Galaxy is, say, only a million years more advanced than we are, and has not yet colonised our part of the Galaxy, in which case we are at point C.

In contrast with the Steady State model, in which stars are occupied randomly, galactic real estate is occupied by colonies in an expanding bubble of space centered on the first civilisation’s planet of origin. Two or more such expanding bubbles may appear, but only if two or more independent civilisations make the breakthrough to space colonisation within about 20 million years of one another – unlikely to occur within any one galaxy. Once the Big Bang is complete the number of stars occupied at any one time is a large fraction of the total, including virtually all main-sequence stars, thus certainly over 0.9 of the total.

Collapsed civilisations are likely to be recolonised from other colonies. In fact, it is possible for every single civilisation to collapse (just as every individual in a population dies), but so long as each civilisation despatches on average more than one colony during its lifetime, the galactic population of civilisations continues to grow.

The original civilisation quickly leaves its planet of origin and adopts a new, space-colony mode of living which allows its offshoots to prosper at all stable stars with orbiting planetary or asteroidal material. This both reduces the distances of interstellar journeys for such a species, and pre-adapts them to living conditions on multi-generational voyages. But all civilisations which evolve after the Big Bang (unless they appear almost simultaneously with it, around point C on the diagram) grow up in an environment dominated by their local colony of the original civilisation.

Their access to advanced technologies, including space transport, would presumably be analogous to the access which a non-developed tribal people on present-day Earth may or may not have to the developed world’s existing consumer technology, financial and transport networks.

The Drake equation

It hardly needs to be emphasised that Drake’s famous equation, with its string of probability factors multiplied together, only applies in the case of the Steady State model.

If, on the other hand, interstellar colonisation is the natural outcome of the appearance of a technical civilisation in an undeveloped galaxy, then the Drake equation takes the following simple form:

  1. If the time is less than TB (the time at point B on the diagram), then the number of civilisations is N = zero;
  2. If the time is equal to TB then N = one;
  3. From time TB to that time plus a few tens of millions of years, N is rapidly growing;
  4. From time TB plus a few tens of millions of years onward for the rest of the lifetime of the Galaxy, N is more or less equal to the total number of stars useful to technological civilisation.

Given the relative brevity of the Big Bang stage, as well as the many unknowns governing the expansion of civilisation from one star to many, it is not worthwhile attempting any greater precision in estimating a precise growth curve for stage 3 above.

The Hybrid model

It is possible to combine these two contrasting models in a single Hybrid model if some emerging technological civilisations get as far as radio astronomy but do not achieve interplanetary and hence interstellar space colonisation.

Steady state diagram

Diagram 3 shows schematically how many civilisations exist at any one time in the Galaxy on the Hybrid model. If our own civilisation collapses before we establish viable extraterrestrial colonies, then we are at point A; if we do succeed in spreading into space, then we are at point B.

In either case, we are unlikely to find interstellar conversation partners. The level of development at which we have radio astronomy but not space colonisation is not in itself a long-term sustainable level, I would argue, but rather an unstable intermediate stage. Having got as far as radio astronomy, a civilisation will either complete the transition to a space-based civilisation within a few centuries, or completely collapse.

This means that the longevity of a society which tried to stabilise itself at that level would be very small, certainly less than 1000 years; the number of such civilisations present at any one time would be correspondingly small; and the distance over which any messages would have to be exchanged correspondingly large, making successful communication correspondingly unlikely.

(If there were as many as 1000 civilisations at any one time, i.e. N = L in Drake’s equation after all the other factors have roughly cancelled each other out, and 1011 stars in the Galaxy, then for an average interstellar spacing of 5 light-years the average spacing between active civilisations would be 2000 light-years, and the waiting time between sending a question and receiving a reply would be greater than the lifetimes of both the transmitting and receiving civilisations.)

What about point D on the diagram? If our own location was at this point, it would correspond to a scenario in which the Galaxy is dominated by one or more alien intelligences, but we remain completely unaware of their existence. While this is logically possible, it is unsatisfactory from a scientific point of view because it introduces complexity into our picture of the universe which is not necessary to explain the observations. Rather than postulate the existence of an advanced alien culture, and then postulate a mechanism which hides it from our view and substitutes the picture of a Galaxy apparently untouched by any kind of civilisation, it is more economical to assume that the apparent loneliness of the Galaxy is real, until evidence to the contrary turns up.

The question of longevity

Given modern fears about nuclear war, peak oil, environmental degradation, social degeneration, technological disaster, climate change and terrorism, spiced with a strong dose of post-colonial guilt and self-loathing, for many people it goes against the grain to think of an industrial civilisation like ours as something which might become a permanent feature of the universe.

What is the lesson from the past evolution of life? Firstly, it must be acknowledged that industrial mankind is as different from our pre-industrial forebears as they were from single-celled Precambrian organisms. Unless one is to maintain that science and technology are somehow unnatural, an aberration from the God-given natural order, then the facts must be acknowledged: a new type of life has emerged with capabilities never before seen. They include the abilities to access other heavenly bodies, and to digest the raw materials found there, neither of which was possible before, except in the marginal case of small numbers of bacteria being randomly exchanged between Earth and Mars.

The pattern from evolution is that each level of biology has given rise to a higher level founded on it: thus prokaryotic cells produced eukaryotic cells produced multicellular life produced technological life – in my own terminology: microbiotagaiabiotatechnobiota.

As each new level of complexity appears, the previous level also persists in symbiosis with it. Furthermore, whereas even bacterial life could not originally have hoped to outlive the death of the Earth (and of Mars) when the Sun approaches its red giant phase, providing that our civilisation fulfils its potential then those less complex organisms will, along with ourselves, continue to live and prosper long after the death of the Sun.

The pattern therefore suggests not only that our technological civilisation will produce some kind of successor at a higher level of complexity, but also that it will not die out once it has become properly established.

Clearly our society is still going through a period of rapid transition, and cannot possibly be regarded as well established yet. It is still experiencing technological and social revolutions, it has not yet reached its final form, and it is still a monoculture. Only once it has technologically matured and begun to diversify at a variety of different Solar System locations, and even more so at a variety of nearby stars, will it be possible to say that civilisation has finally arrived.

Once it has arrived in full flower, the more dynamic branches of it will certainly spread, because regardless of the precise impulse at work that is what life has always done.

Consider the question: where can one find bacteria on Earth? The answer is: nowhere, for an unknown period of time early in Earth’s history. Then there is a Big Bang, a relatively brief explosion of bacterial life, and thereafter the answer is: everywhere.

Our industrial society has yet to experience the equivalent of that Big Bacterial Bang, or of the Cambrian explosion of 550 million years ago when a plethora of new and diverse multicellular forms emerged and went their own ways. That will require our descendants to expand on an interplanetary and ultimately an interstellar scale. But when they do, or when some other civilisation does if we don’t make it, and if life develops in the future as it has in the past, then civilisation will certainly become a ubiquitous and universal feature of the Galaxy for as far into the future as it is possible for us to glimpse.

Answering Fermi’s question

I commented on this at length in AE last December, and this posting was also reproduced on the I4IS blog.

In brief: the question is why civilisation did not arrive before now, with a starting point elsewhere than on Earth, given that the stelliferous universe with earthlike planets is about three times older than the Solar System.

The reason why people have such a problem with this, and refer to it as a paradox, is because they are wedded to the traditional view since Darwin that life evolved from chemistry on Earth, whether in a “warm little pond”, a piece of damp clay or a hydrothermal vent. If that was the case, then since it evolved within about 300 million years after the end of the late heavy bombardment, it should have done so on many other planets, and billions of years earlier.

But Robert Zubrin makes the point that there is a huge complexity gap between the simplest bacterium known to science and the most complex molecule that can be synthesised by shaking up raw materials in a test-tube. Some proto-bacterial form of life must have preceded life as we know it. But there is no evidence of proto-bacterial life on Earth.

This to my mind is strong evidence that, contrary to the generally accepted view, life does not evolve from non-life on Earth-like planets. Those who believe it does suggest that the proto-organisms that arise subsequent to the evolution of bacterial cells get quickly eaten by those cells. How plausible is this? Single-celled organisms do not get removed from the environment by multicellular ones; they are ubiquitous. Would not proto-cells also be found everywhere, and be as vastly more numerous than cells, as cells are more numerous than multicellular animals? Biologists would then observe a continuous chain of organisms all the way down to the smallest self-reproducing molecule.

The obvious alternative scenario for the origin of life involves it first emerging in a microgravity environment in something like a comet nucleus, and doing so only extremely rarely. There has been some interest in experimenting with protein growth in microgravity on the International Space Station: maybe the lack of gravity is essential for one step in the development of the first self-replicating molecule. But even if the first instance of abiogenesis needs a more terrestrial type of world to take place, it could still happen so rarely that it has not yet had time to produce multicellular life on a single world, but through its impact dispersal into space giving Earth an early start.

This kind of scenario decouples the initial emergence of life from its subsequent evolution to multicellular forms, allows a period perhaps 100 times longer for that initial jump in complexity to occur, explains why proto-bacteria have never been found on Earth, and furthermore adds in the requirement for a low-probability space transfer before evolution towards multi-cellular forms can begin, pushing the Big Bang of technobiotic life to the right on the diagram.

But not too far to the right. For all that the accepted age of the universe of 13.7 billion years seems to us to be unimaginably ancient, on its own terms the universe is still young. Judged by the lifetimes of its longest-lived stars, the red dwarfs, the universe will continue to contain stars and planets as we know them for a period on the order of tens of trillions of years to come, though the brighter stars will fade long before then. If the universe was a human being, it would still only be about a month old.

Another factor may play a part. Carl Sagan has described how the modern alien encounter/alien abduction mania perpetuates the phenomenon of encounters with angels and demons and with the Virgin Mary in earlier centuries. Could the flood of speculations about alien civilisations – where are they? are they hostile or friendly? – be the modern equivalent of the search for God? Do people still yearn to submit to an Overlord (the name given to the aliens in Arthur C. Clarke’s Childhood’s End), whether beneficent, or intent on our punishment?

Until we find any evidence of alien intelligence, the most parsimonious explanation is that there isn’t any where we have looked, that there is no invisible dragon in my garage (Sagan’s image). So we must look further, before it will be possible to use observations to rule out any of the models described here.


Ian Crawford, “Where Are They?”, Scientific American, July 2000, p.28-33.

Guillermo Gonzalez, Donald Brownlee and Peter D. Ward, “Refuges for Life in a Hostile Universe”, Scientific American, October 2001, p.52-59.

Michael A.G. Michaud, Contact with Alien Civilisations (Copernicus, 2007).

Carl Sagan, The Demon-Haunted World: Science as a Candle in the Dark (Headline, 1997); Contact (Century Hutchinson, 1986).

Nathan Taylor, “Avoiding ‘Sagan Syndrome’: Why Astronomers and Journalists should pay heed to Biologists about ET”, posted 25 November 2013.

Robert Zubrin, “Interstellar Panspermia Reconsidered”, JBIS, vol.54, no.7/8 (July/August 2001), p.262-269.